The present invention relates to a lambda oxygen sensor for detecting concentration of oxygen in an exhaust gas from a vehicle or the like, and more particularly to a lambda oxygen sensor with which the enhancement of reliability of a preheating control and the like can be acquired.
In a lambda oxygen sensor where it is necessary to maintain a temperature of a sensor element at an active temperature, it is necessary to perform a preheating control of a heater before performing an operation of detecting concentration of oxygen. Various control methods and devices have been proposed and put into practice aiming at the enhancement of reliability, stability and the like of such a preheating control.
For example, there have been disclosed techniques including a technique where correlation between a heater resistance value and a temperature is focused, the heater resistance value is detected, and it is determined whether or not a temperature of the whole sensor arrives at a sufficient temperature for performing a detection operation based on the resistance value (see JP-A-2009-288082 and the like, for example).
The supply of electricity to a heater for a lambda oxygen sensor often adopts a method where a pulse voltage having a predetermined pulse width is applied to the heater at a predetermined repetition cycle. In such a method, a pulse width is determined based on a premise that a pulse voltage waveform is a square shape. In an actual operation, however, a delay occurs in a rise time or a fall time of a pulse because of a delay of a signal in a drive circuit, an electric characteristic of a semiconductor element or the like.
Accordingly, an effective voltage which a voltage to be applied to the heater is required to be cannot be maintained thus giving rise to a possibility that shortage or excess of preheating is brought about.